The present invention relates to a method for measuring the concentration of trace amounts of OH groups that are contained in quartz glass, especially in quartz glass to be used in preforms of optical fibers, cells for use in spectroscopy and lamps.
In particular the invention relates to a method for measuring a low concentration of OH groups (less than 1 ppm) at high precision.
Much attention is paid on the content of OH groups that are incorporated in quartz glass, for instance, in optical fibers, because OH groups increase the transmission loss of light. Conventionally, an infrared spectrophotometer has been used to measure the concentration of the OH groups contained in quartz glass, and by measuring the height of the absorbance peak in the near infrared region attributed to the OH groups, the concentration is calculated in accordance with the following equation:
Csample=xcex94A/Lxcex5xc3x97(MH+MO)/xc3x97103xe2x80x83xe2x80x83(1),
wherein Csample represents the concentration of OH groups that are present in the sample test piece (in wtxc2x7ppm); xcex94A represents the peak height (ABS); L represents the length (cm) of optical path; xcex5 represents the absorption coefficient (1xc2x7molxe2x88x921xc2x7cmxe2x88x921),
MH represents the atomic weight of hydrogen; MO represents the atomic weight of oxygen; and represents the density (gxc2x7cmxe2x88x923) of quartz glass.
However, in the measuring method above, the intensity of the absorbance peak decreases with decreasing concentration of OH groups making the accurate measurement of the concentration difficult. Thus, even if the absorbance peak having the highest sensitivity at a wavelength of 2720 nm is used, the detection limit of the concentration for an optical path length of 1 cm is said to be about 1 ppm. Accordingly, since the height of the absorbance peak increases in proportion to the length of optical path, it has been proposed to make the optical path longer, but a longer optical path length results in an increase in absorbance for the longer wavelength side due to the increase in base absorption of the quartz glass. This led to a loss in measurement precision due to the baseline inclining upward with increasing wavelength. Although it is possible to circumvent the interference of this base absorption by selecting the absorbance peak at a wavelength of 1380 nm attributed to OH groups, it was still unfeasible to perform measurements with high precision if a conventional test specimen was used, because the sensitivity was lowered to about {fraction (1/160)}. In the light of such circumstances, the concentration of OH groups that are present in trace quantity in quartz glass has been determined by preparing optical fibers of the quartz glass, and by then selecting the less sensitive absorbance peak at a wavelength of 1380 nm by taking advantage of the superior light transmission characteristics and the increase in optical path length.
However, in the conventional method described above, the measurement was made possible only after the quartz glass is drawn into an optical fiber, and it was impossible to determine the concentration of the OH groups that are incorporated in trace quantities in the preforms. Furthermore, it was not feasible to measure the concentration of the OH groups contained in the quartz glass that are used in applications other than optical fibers such as the cells for use in spectroscopy, quartz glass tubes for lamps, etc.
In the light of the circumstances above, an object of the present invention is to provide a simple method for measuring the concentration of OH groups that are contained in trace quantity in quartz glasses of any type, yet at high precision.
Furthermore, another object of the present invention is to provide a method for measuring the concentration of OH groups that are present in the quartz glass at a quantity of less than 1 ppm, yet at a high precision.
To achieve the objects above in accordance with the present invention, there is provided, in measuring the concentration of OH groups contained in quartz glass by means of infrared spectroscopy, a method for measuring the concentration of OH groups that are contained in trace quantity in quartz glass, which comprises preparing a set of test pieces comprising a reference test piece from a blank of a quartz glass body whose OH group content is known and a sample test piece from a quartz glass body whose OH group content is to be measured, whereby the test pieces each have two planar planes faced to each other; setting the sample test piece and the reference test piece in an infrared spectrophotometer; successively irradiating perpendicular to one of the two planar planes an incident infrared radiation in a wavelength region of approximately 2500 nm to approximately 2950 nm, while simultaneously detecting the outgoing radiation spectrum from the other plane; obtaining the difference of the outgoing radiation spectrum of each of the test pieces; selecting the absorbance peak assigned to OH groups at a wavelength of 2720 nm to obtain the peak height thereof; and calculating the concentration of OH groups from the peak height in the sample test piece.
For calculating of the concentration of OH groups that are present in a trace quantity in the sample test piece the following equation is suitable:
Csample=Cblank+xcex94A/Lxcex5xc3x97(MH+MO)/xc3x97103xe2x80x83xe2x80x83(2),
wherein, Csample represents the concentration (wtxc2x7ppm) of OH groups that are present in the sample test piece; Cblank represents the concentration (wtxc2x7ppm) of OH groups that are present in the reference test piece; xcex94A represents the peak height (ABS.); L represents the length (cm) of optical path; xcex5 represents the absorption coefficient (1xc2x7molxe2x88x921xc2x7cmxe2x88x921), MH represents the atomic weight of hydrogen; MO represents the atomic weight of oxygen; and  represents the density (gxc2x7cmxe2x88x923) of quartz glass.
In the measurement method according to the present invention, the baseline inclined by the presence of base absorption can be set horizontally by calculating the concentration of OH groups that are present in a trace quantity in the quartz glass by obtaining the difference spectrum. Furthermore, by selecting the highly sensitive OH groups absorbance peak at the wavelength of 2720 nm, the concentration of OH groups that are present in a trace quantity of lower than 1 ppm can be obtained. Moreover, since the method does not require preparing an optical fiber therefrom, the concentration of OH groups that are present in trace quantity can be measured on any type of quartz glass bodies.
The method according to the present invention requires a quartz glass body to use as a blank whose OH group content is known. However, to measure the concentration of OH groups contained in the sample test piece at high precision, it is preferred to use a blank test peace made from a quartz glass body free from OH groups, i.e., a quartz glass body having an OH content of xe2x80x9c0xe2x80x9d percent. xe2x80x9cA quartz glass body free from OH groupsxe2x80x9d signifies a quartz glass body whose part is made into a fiber and whose spectrum is measured to confirm that there is no absorption at 1380 nm ascribed to the presence of OH groups. By using this quartz body, the concentration of OH groups that are present in trace quantity can be obtained with high precision.
The precision of measurement can be increased by providing two planes disposed faced to each other and through which the light is passed in such a manner that are placed in parallel with each other and providing them as mirror finished planes, or polished planes or cut planes coated with an oil for matching the refractive index or a reflection preventive agent, such that they yield a center line average roughness Ra of 10 xcexcm or less. A reflection preventive agent is an organic substance free from OH groups and having an effect of reducing the reflection index. Furthermore, an increase in planar precision enables measuring OH group concentration in a further lower range.
The optical path length of the reference test piece and the sample test piece is preferably in a range of from 2 to 50 cm, more preferably, in a range of from 2 to 8 cm. If the length is less than the aforementioned range, it becomes difficult to achieve the required measurement precision, and if the optical path length exceeds 50 cm, the measuring region becomes dark as to make it difficult to make measurements with high precision. In practice, the optical path length is preferably in a length capable of setting it inside the sample chamber of the infrared spectrophotometer, i.e., a length of from about 2 to 8 cm.
If the difference in optical path length for the reference test piece and the sample test piece becomes larger, the inclination of the base line becomes too large as to impair the measurement precision. Accordingly, to perform measurements with high precision, it is effective to decrease the difference in optical path length. Although it is preferred to shorten the difference in optical path length as much as possible, a difference of less than 1% (relating to the optical path length of the reference test piece) makes no problem in practice.
Preferably before setting the sample test piece and the reference test piece in the infrared spectrophotometer, the difference in fictive temperature of the sample test piece and the reference test piece is set to be within 200xc2x0 C. The difference in fictive temperature between the reference test piece and the sample test piece generates a mismatch between the baseline at the longer wavelength side and the shorter wavelength side. Unlike the case attributed to the difference in optical path or the difference in the planar state, this mismatch leads to a shift of the baseline in the longer wavelength side in parallel with the direction of the longitudinal axis. Accordingly, this mismatch greatly affects the precision of the measurement. The reason for this is presumed to be attributable to the difference in the basic absorbance between the reference test piece and the sample test piece. It is reported in Anand Agarwal, Kenneth M. Davis, Minoru Tomozawa, xe2x80x9cA simple IR spectroscopic method for determining fictive temperature of silica glassesxe2x80x9d, Journal of Non-Crystalline Solids, 185 (1995) 191 -198, that the peak position at 1122 cmxe2x88x921 shifts with the change in fictive temperature. The present inventors presume that the peak shift at ca. 3366 cmxe2x88x921 (equivalent to 2970 nm), i.e., the triple overtone of 1122 cmxe2x88x921 according to the reported value in the literature above, shows the change in basic absorbance. According to the understanding above, the peak position shifts to the side of lower wavelength (longer wavelength) with elevating fictive temperature. Thus, when the reference test piece having a lower fictive temperature and the test piece having a higher fictive temperature are measured, it results in an increased baseline at the longer wavelength side, and in a declined baseline in case the fictive temperatures are in the reversed relation. This is confirmed in the measurement performed in the present invention.
To measure with higher precision, it is effective to reduce the difference in the fictive temperatures for the two test pieces. It is preferred that the difference in fictive temperatures is reduced as much as possible. However, a difference in fictive temperatures within 80xc2x0 C., preferably 50xc2x0 C., makes no practical problem.
To match the fictive temperature of the reference test piece and that of the sample test piece, the test pieces are each subjected to a heat treatment under the same conditions; otherwise, the fictive temperature of the reference test piece is set to be the same as that of the sample test piece. In particular, the method of subjecting the reference test piece alone to the heat treatment is preferred, because it prevents the fluctuation in the concentration of OH groups from occurring on the sample test piece to be measured.
The fictive temperature is measured by using a Raman spectrophotometer. More specifically, a small piece of a synthetic quartz glass is prepared as a comparative sample; for example, sample 1 obtained by water-quenching a piece once heated at 1200xc2x0 C. for a duration of 2 hours, sample 2 obtained by water-quenching piece heated at 1000xc2x0 C. for a duration of 20 hours, sample 3 obtained by water-quenching piece heated at 900xc2x0 C. for a duration of 120 hours, and sample 4 obtained by water-quenching piece heated at 800xc2x0 C. for a duration of 1200 hours are prepared, and each of the samples is subjected to Raman spectroscopy in the wavelength range of from 150 to 650 cmxe2x88x921 to obtain the following three peaks:
150 -650 cmxe2x88x921 (W1, peak area AW1);
470-520 cmxe2x88x921 (D1, peak area AD1); and
580-640 cmxe2x88x921 (D2, peak area AD2).
Then, from the three peak areas obtained above, the area ratio I is obtained in accordance with the following equation:
I=AD2/(AW1xe2x88x92AD1xe2x88x92AD2)xe2x80x83xe2x80x83(3)
Then, the relation between the thus obtained value I and the fictive temperature is shown in a graph to provide a standard line (calibration line). Thus, there can be shown a method of obtaining the fictive temperature from its xe2x80x9cIxe2x80x9d for a sample whose fictive temperature is unknown.
By using the measurement method according to the present invention, the present inventors detected the concentration of OH groups present in trace quantity as low as 0.005 ppm. Presumably, OH groups present at a still lower concentration can be detected by using measurement equipment with higher performance, or by improving the preparation of samples.